WS-ReliableMessaging: Where Are Message ID's

It's been a long day, and I might be missing something. I just glanced
at the GXA WS-ReliableMessaging spec [1] and noted that in the
definition of message sequence, there is a message number but no message
ID. From p.10 of spec:

Sequence definition:

<wsrm:Sequence ...>
   <wsu:Identifier> [URI] </wsu:Identifier>
   <wsrm:MessageNumber> [unsignedLong] </wsrm:MessageNumber>
   <wsrm:LastMessage/>?
   <wsu:Expires> [dateTime] </wsu:Expires>?
   ...
</wsrm:Sequence>

Example:

<wsrm:Sequence>
   <wsu:Identifier>http://fabrikam123.com/abc</wsu:Identifier>
   <wsrm:MessageNumber>10</wsrm:MessageNumber>
   <wsrm:LastMessage/>
</wsrm:Sequence>

Yet, in WS-Reliability, we specify a GroupID in the MessageOrder
element:

<rm:MessageOrder xmlns:rm:="http://schemas.fujitsu.com/rm"
      SOAP:mustUnderstand="1">
      <rm:GroupId status="Continue"></rm:GroupId>
      <rm:SequenceNumber>12</rm:SequenceNumber>
</rm:MessageOrder>

Where GroupID is "used to specify the group of messages for which
message ordering is guaranteed."

Can anyone please confirm my observations? It seems very odd that
message ID's do not appear in message sequences in the
WS-ReliableMessaging spec.

Kind Regards,
Joe Chiusano
Booz | Allen | Hamilton
[1] http://msdn.microsoft.com/ws/2003/03/ws-reliablemessaging